1. Field of the Invention
This invention relates to a method for applying FM screening to a digital image. More particularly, it relates to a novel method for applying FM screening to a digital image, permitting editing of an FM screened digital image "on-screen", before the FM screened image is directed to an imaging device.
2. Description of Prior Art
FM screening (frequency modulated screening), also referred to as stochastic screening, is a process of randomly applying very small dots of the same size to a digital image. The relative size of the dots used in FM screening remains constant while their position changes; distances between the dots determine different color tones. FM screening processes of the prior art apply the FM screening at an imaging device which immediately thereafter prints the image through an output. of the imaging device.
FM screening is different from a process known as AM screening (amplitude modulated screening) which utilizes perfectly geometric dots, such as round, square, diamond, elliptical, and other geometric dots, which change in size but remain in a constant position. AM screening employs conventional half-tones to acquire different shades of color. Conventional halftones, also known as middle tones, of any color are produced by varying the size of the dots being used, hence AM screening, changing the "amplitude" of the dot. The simplest example of conventional half-tones is seen in black and white offset printing, wherein conventional half-tones create the illusion of gray tones being present in a picture. By varying the size of the black dots to allow a larger area of the white paper to show through in a picture, gray tones are produced. FM screening does not utilize conventional half-tones to produce different color tones, but instead varies the position or "frequency" of the dot to produce different color tones, hence frequency modulation screening--FM screening.
FM screening can produce a printed image having superior characteristics over an original digital image and to printed images utilizing processes such as AM screening or other standard four-color printing processes. Examples of the superior characteristics include better definition and resolution, enhanced color and hue, smoother gradations of tones, and an ability to retain delicate qualities of an original image. Another advantage of FM screening includes the elimination of moire patterns, an interference problem that occurs when printing four conventional half-tones on top of each other, wherein the dots of the four conventional half-tones do not fall at the precise right angles. Because FM screening places the dots randomly and does not use conventional half-tones to produce different color tones, no interference patterns are created.
Although FM screening can produce an image with superior characteristics over other standard printing processes, there are some inherent disadvantageous with the current FM screening processes. The known FM screening processes apply the FM screening to a digital image in a single file at the imaging device, utilizing a software program compatible with the imaging device. The software program is loaded into the RIP (Raster Imaging Process) of the imaging device. The software in the RIP seriously limits what can be done to an FM screened image. The prior art does not allow the FM screening to be applied to the individual color separations of the image. Further, editing an FM screened image "on-screen" can not be done in the prior art since the FM screening is applied immediately before printing. The end result of the FM screened image is determined after the image has been printed through the output of the imaging device; there is no way to provide a proof without printing. Consequently, many printed FM screened images are discarded because the desired results are not attained. The user is forced to return to the original digital image, having no FM screening applied, manipulate that image, and finally send the image to the imaging device, allowing the FM screening to be applied at the RIP, thereafter printing another FM screened image through the imaging device output. If the desired results are not satisfactory, the printed image is again discarded and the steps are performed until the desired result is produced. Therefore, manipulation of the digital image using the prior art FM screening processes is simply "trial and error." One can easily see the increase of costs due to materials being discarded as well as the operator time that is wasted attempting to render the desired printed FM screened image using known FM screening processes.
Another disadvantage of the FM screening prior art involves dot gain of FM screen dots. Dot gain is defined as the difference between the actual printed density of a color and the specified density. Because FM dots are smaller, dot gain is greater in FM screening processes. Essentially, smaller dots spread out more in relation to their size. The problem of dot gain in the prior art can be solved by compensating for the gain at the imaging device. This involves. calibrating the imaging device for proper FM screening output, an extra cost most likely passed on to the consumer by the operator or owner of the imaging device.
Yet another inherent disadvantage of the prior art is in that the software loaded in the RIP determines the specific shape and density of the dot used in the FM screening process. The specific shape and density of the FM dots are determined by the manufacturer of the software program when it is written. The user of the software program has no ability to customize shape and density of the FM dots. Since the software loaded in the RIP is written to be compatible with a particular imaging device, the FM screening capabilities of individual imaging devices are seriously limited by the FM screening software.
There is a need for a novel method for applying FM screening to a digital image which will allow a user to manipulate the image after the FM screening has been applied, and before the image is printed by the imaging device. The novel method should allow the operator to perform editing of the FM screened image "on-screen." Further, the novel method should eliminate the need for the software program loaded in the RIP of the imaging device.